The way a planet is tilted on its axis of rotation relative to its orbital plane around a star, the axial tilt, could be the key to the emergence of complex life.
Until proven otherwise, Earth is the only planet known to host a biosphere, and what's more, an incredibly rich biosphere complex. This is why astrobiologists focus their work on the potential for extraterrestrial life on exoplanets similar to our own:small, rocky worlds inhabiting the habitable zone of their star, where temperatures would be neither too cold nor too hot. to allow the maintenance of liquid water on the surface.
These points obviously seem important, but other factors favoring the emergence of life must probably also be taken into account. The presence of a magnetic field, for example, seems essential to protect the planetary atmosphere from stellar winds. The orbital eccentricity and the type of planets evolving in the same system could also be decisive.
In a new study, Stephanie Olson and her team at Purdue University have focused more on the evolution of complex life that depends very much on oxygen produced by photosynthetic life on Earth. This work was presented at the Goldschmidt Geochemistry Conference 2021.
Life on Earth is based on carbon. We know that the first terrestrial life forms, which emerged probably about four billion years ago, got their carbon from carbon dioxide (CO2) molecules extracted from the atmosphere through solar energy (photosynthesis) . Each reaction of fixation of a carbon atom then releases a molecule of dioxygen (O2), which is therefore for these living organisms (cyanobacteria) only a toxic waste.
Over time, photosynthetic organisms have become numerous enough to manufacture large amounts of oxygen. About 2.4 billion years ago, the oxidation of the iron that the ocean contained at that time no longer became sufficient to eliminate them. Dioxygen has therefore spread through the oceans and the atmosphere , ultimately upsetting the physico-chemical balance of our planet.
It was from this time, known as the Great Oxidation (or Great Oxygenation), that terrestrial (multicellular) life forms could come into existence and grow.
As part of this new work, Olson and his team therefore sought to determine the conditions that played a key role in this famous "boom" of cyanobacteria on Earth. Indeed, without this boom, no oxygenation, and without oxygenation, no complex life.
The researchers relied on computer models by integrating and testing different factors to determine the behavior of cyanobacteria. According to these models, the slowing of the Earth's rotation, the lengthening of days and the formation/migration of continents could have influenced the transport of nutrients in the oceans in such a way as to favor the growth of organisms producing oxygen. .
Still according to these models, the axial inclination would also be a key factor. As a reminder, the axis of the Earth is not exactly perpendicular to its orbital plane around the Sun. In reality, it is tilted at an 23.5 degree angle . However, it is this inclination that influences seasonal variability . Over the seasons, changes in temperature then favor the development of convective currents and the mixing of nutrients, and therefore the proliferation of oxygen-producing organisms.
In contrast, it's about finding the "right balance" . Uranus, for example, is tilted 98 degrees from the perpendicular. Such an extreme tilt would cause seasonality that might be too extreme for life. Conversely, a small tilt may not produce enough seasonality to encourage the right level of nutrient availability.
Thus for the authors, this axial inclination neither too extreme nor too small is therefore another parameter to take into account to help reduce our planetary targets in exobiology. Naturally, it is possible for life to emerge outside of the parameters we know here on Earth, but again, our "pale blue dot" is so far the only world known to harbor life. Therefore, we should model our research accordingly.
